Protein S is a vitamin K-dependent anticoagulant protein which circulates in plasma at a concentration of about 25 μg/ml with a half-life of about 2 days. In normal plasma, 60% of protein S binds to C4b-binding protein (C4b-BP) non-covalently in a 1:1 ratio with high affinity. Protein S that is bound to C4b-BP is inactive. The remaining 40% of protein S exists as free protein in plasma and is believed to be the physiologically active anticoagulant form which acts on the cell membrane surface as a cofactor for activated protein C (APC). APC degrades the active forms of procoagulant factors V (FVa) and VIII (FVIIIa) through specific proteolytic cleavage, thereby reducing thrombin generation and prolonging clotting time. Protein S binds to APC and acts as a cofactor and increases the cleavage rate of factors Va and VIIIa. Protein S also exerts a direct inhibitory effect on the prothrombinase complex by binding to factor Xa and to factor Va, and thus impairing prothrombin activation.
Protein S deficiency may be hereditary or acquired. Acquired deficiency may be observed during pregnancy, oral anticoagulant therapy, oral contraceptive use, in liver disease, in newborn infants, as well as in other clinical conditions. Because Protein S is a vitamin K-dependent protein, its concentration decreases during treatment with oral anticoagulants. With a half-life of two days, the rate of decrease for protein S levels is much lower than for protein C and factor VII, which have half-lives of several hours. A representative normal range for total protein S is 70-140%. Considering 25 μg/ml as the mean concentration, this corresponds to a range of 15-35 μg/ml. Protein S levels may be influenced by sex hormones such as estrogens. Pre-menopausal women have lower values than men and post-menopausal women. Significantly lower mean values of total and free protein S are found in pregnant women (from 25 μg/ml to 15 μg/ml) and women using oral contraceptives (from 25 μg/ml to 18 μg/ml). Acquired and congenital protein S deficiency is associated with an increased risk of thrombosis (e.g., deep vein thrombosis) due to a decrease of blood anticoagulant potential. Hereditary protein S deficiencies include familial thrombophilia.
The current subclassification of protein S deficiency into three types was recommended by the Scientific Standardization Committee of the International Society on Thrombosis and Haemostasis (ISTH) in 1992. Type I is characterized by low levels of total and free protein S with a decrease in functional protein S activity. Type II is characterized by normal levels of total and free protein S with a decrease in functional protein S activity. Type III is characterized by normal levels of total protein S and a low level of free protein S, with a decrease in functional protein S activity.
Antigenic (immunological) assays measure the concentrations of either total or free protein S, depending on the antibody and/or procedure used. Functional assays for protein S measure the biological activity of protein S. Since protein S bound to C4BP does not have anticoagulant activity, it is important to know the concentration of the free protein S that is available to act as a cofactor for APC. Free protein S can be quantitatively determined in several ways, for example, the C4BP-protein S complex may be precipitated with polyethylene glycol and the concentration of free protein S in the supernatant may be determined. Alternatively, free protein S may be directly measured by capturing free protein S with immobilized C4BP (e.g., C4BP bound to wells of a microplate) and quantitating with antibody (Coaliza® Protein S-Free Assay, Chromogenix-Instrumentation Laboratory Company SpA, Milan Italy).
Protein S activity does not always correlate with protein S levels in a plasma sample. For example, a free protein S concentration obtained using an antigenic method correlates well with functional activity for patients with Types I and III but not Type II protein S deficiency for a number of reasons. First, antigenic assays measure both fully carboxylated (active) and non-carboxylated (inactive) forms of free protein S. Second, the functional protein S assays are complicated by the presence of both the free and complexed forms in plasma. Thus, antigenic assays can overestimate the level of functional protein S. For example, an antigenic assay of plasma from patients receiving warfarin will give higher values than those obtained using a functional assay. It is therefore important that both a functional and an antigenic assay be performed to screen patients at risk of thrombotic disease for protein S deficiency (i.e., deficient protein S levels and/or deficient protein S activity).
In some functional protein S activity assays, the effect of free protein S as a cofactor to APC is determined. These assays are predominantly coagulometric and measure the prolongation of the clotting time due to free protein S activity as a consequence of the degradation of FVa and FVIIIa by APC. APC-cofactor methods for free protein S activity have traditionally included the prothrombin time (PT), the activated partial thromboplastin time (APTT) and factor Xa-based methods, described below. In addition, free Protein S also exerts an APC-independent anticogulation activity through direct binding to factor Va, factor Xa and factor VIII. An assay of the APC-independent anticoagulant activity of protein S has been developed in which the clotting time is determined in the presence and absence of a polyclonal protein S antibody.
Protein S functional assays may be based on the prothrombin time (PT). The cofactor activity of protein S is confined to the APC-dependent degradation of factors Va and VIIIa. Originally, a method was developed for characterization of purified protein S, which was later followed by a functional test for determining protein S in plasma. (Walker (1984) Sem. Thromb. Hemost. 10:131-38). Protein S activity is determined by mixing a plasma sample with protein S-deficient plasma. The stimulating effect of protein S on the anticoagulant activity of APC is measured by observing clotting time following the addition of thromboplastin (Tissue Factor) and calcium ions to a plasma sample with and without the addition of exogenous APC or exogenous protein C activator (PCA). PCA may be isolated from snake venom from Agkistrodon contortrix, which is known under the proprietary name Protac® C (Pentapharm, Basle, Switzerland). A resolution of 40-50 seconds is obtained between 0 and 100% protein S.
Protein S functional assays alternatively can be based on the prolongation of activated partial thromboplastin time (APTT) due to exogenous APC or exogenous PCA.
The standard APTT reaction begins by adding a surface-activating agent (e.g., Kaolin, silica, ellagic acid) and a phospholipid preparation to a plasma sample, thereby achieving maximum activation of factor XI. Calcium is then added to activate the coagulation cascade and the time for clot formation is determined.
In APC resistance assays (e.g., COATEST and COATEST F), two APTT reactions are performed, one in the presence of APC (or PCA) and the other in its absence. The result can be calculated either as a prolongation of clotting time or as a ratio between the clotting times in the presence or absence of APC (or PCA). The APTT reaction without the addition of APC (or PCA) should be within the normal range of 25-40 seconds.
However, the cut-off value for all assays known to date varies between laboratories, instruments, reagent handling and other preanalytical variables. For this reason, APTT and PT assays typically require that a normal control sample be run in parallel. In such cases, the clotting time and/or clotting time prolongation of the patient sample is compared to that of the normal control sample or samples of known protein S content.
Other protein S assays include FXa-based methods, wherein coagulation is triggered by factor Xa in the presence of calcium ions and phospholipids. Originally, undiluted plasma was used. (Comp (1984) J. Clin. Invest. 74:2082-2088.). This was later replaced by methods to minimize interference by prothrombin levels in the plasma, allowing dilution of test plasma and providing close to 100 seconds resolution between 0 and 100% protein S. (Wiesel et al. (1990) Thromb. Res. 58:461-468.) In one variant of the method, free protein S in the test plasma is first adsorbed on an insolubilized monoclonal protein S antibody. (D'Angelo et al. (1988) J. Clin. Invest. 81:1445-1454). Factor Xa has also been used as a trigger in a system utilizing purified components. Dahlback (1986) J. Biol. Chem. 261:12022-12027).
A prothrombin time method is described in U.S. Pat. No. 5,726,028. The assay uses Thromborel S®, a tissue factor/phospholipid preparation from human placenta and protein C activator. The endogenous protein C in the sample is activated by the protein C activator and forms with protein S active APC/protein S complexes. Clotting is induced by adding calcium ions, and the resulting APC/protein S complexes delay clot formation.
However, this and other assays available generally use crude extracts of tissue factor and phospholipid. In addition, activated protein C, which is also used in the assays is obtained by activating a plasma sample containing protein C with a crude protein C activator, such as snake venom activator, for example. As a consequence of impurities present in these crude reagents, the traditional protein S functional assays suffer from poor reproducibility, low sensitivity and instability.
A need exists, therefore, for a reproducible, sensitive and stable, and functional Protein S assay that, optionally, does not require comparison of the patient results to the results from a normal subject.